Future generation wireless local area networks (WLANs) will be required to provide higher capacity and link speed as well as greater reliability, mobility and adaptivity—even in the presence of severe channel conditions such as multipath distortion and frequency-selective fading. The nature of such severe channel conditions and some existing techniques that attempt to overcome such conditions are summarized below.
Wireless radio communication generally requires a transmitter that modulates data onto radio carrier waves and that transmits the carrier waves to a receiver. The receiver then detects the carrier waves and recovers the data. Often the communications are sent digitally such that the modulated data consists of individual data symbols. To be meaningful, the individual symbols must be synchronized at the receiver to correspond with the temporal data synchronization at the transmitter.
In terrestrial radio broadcasting, wireless systems need to address multipath channels from reflected carrier waves that distort the temporal reception of digital data. For example, transmitted radio waves may reflect off of buildings, mountains and airplanes such that the same signal arrives at a receiver from various paths having different lengths. The different path lengths cause phase differences such that the reflected versions of the signal interfere with each other, an effect known as inter-symbol interference (ISI). ISI can result in severe multipath distortion of the received signal when individual data bits on a carrier wave are transmitted over a time interval T that is significantly less than the delay τ-max between the longest reflected path and the most direct path between a transmitter and a receiver (such that τ-max/T>>1).
The problem of frequency-selective fading is often a result of multipath distortions. An aspect of frequency selective fading is that some frequencies are enhanced while other frequencies are attenuated.
Various techniques have been developed to mitigate the effects of multipath signal distortion and frequency-selective fading. One technique is multiple carrier code division multiple access (MC-CDMA) that is a modulation technique that divides a digital data signal having a high bit rate into numerous parallel bit streams or sub-carriers, each having a much lower bit rate. The lower bit rate results in τ-max/T<1, thus greatly reducing ISI. MC-CDMA uses transmission bandwidth efficiently by densely spacing the sub-carriers in an overlapping, orthogonal arrangement.
However, disadvantages of MC-CDMA techniques include the fact that the lower bit rate requires more sub-carriers, which in turn requires more complex Fast Fourier Transform (FFT) processing steps, which leads to reduced capacity. Complex FFT processing increases system latency and also requires data to be organized in long blocks that add to system overhead.
Other methods for minimizing the effects of multipath distortion include the use of antenna spatial diversity. These methods generally involve a plurality of transmitting antennas in different locations. A receiver then receives multiple signals from the different antennas and calculates multiple transmission paths between the transmitters and the receiver. The signal to noise ratio is then increased by combining the multiple transmission paths coherently. However, disadvantages of these techniques include the fact that each transmitter must be able to transmit with enough power to obtain a minimum signal to noise ratio at the receiver. That is problematic when transmissions occur across a wide area. Also, antenna spatial diversity techniques obviously require additional transmitter and/or receiver equipment.
Multipath distortion also can be minimized through the use of direct, point-to-point transmissions using a narrow transmission beam. But point-to-point transmissions are generally not practical in mobile device applications, particularly in mobile device applications in urban areas where successful signal reception often depends on multiple signal reflections.
Still other techniques to minimize the effects of multipath distortion include adaptive channel equalization techniques and pre-equalization techniques. Adaptive channel equalization techniques can be implemented at a receiver and are useful tools to reduce ISI caused by frequency-selective fading channels in wireless systems. The receiver estimates the nature of an actual signal by subtracting delayed, multipath signals. However, when a transmitted data rate is high and a channel delay spread is long, conventional receiver-based adaptive equalizers become complicated and a system's performance degrades due to imperfect channel estimation and noise amplification.
Recently, pre-equalization techniques, implemented at the transmitter, have been studied as an alternative way to combat frequency-selective fading channels. A brief description of these techniques is given as follows: According to the Lorentz Reciprocity Theorem, the reflections off materials of electromagnetic waves travelling between two points generally demonstrate reciprocity. That is, channel characteristics are equally distorted by waves travelling in either direction. Pre-equalization techniques are therefore used to estimate the distortion of a future signal transmission by first estimating the distortion of a received signal.
Using pre-equalization techniques, a channel condition is estimated at the time of reception of a multipath signal transmitted from a first station. A second station that receives the distorted signal first estimates the actual signal and the multipath components. When the second station next transmits a signal back across the same channel to the first station, the second station pre-equalizes the signal so that the multipath condition at the first station results in the cancellation of the multipath signals, leaving only the desired signal.
Pre-equalization techniques that use static, pre-determined channel measurements have been in use for many years. One example is in twisted-pair Ethernet systems at 100 Mbps and above. However, because of the requirement for a-priori channel measurements, static pre-equalization has been practical only in wired systems and in wireless systems that experience only very slowly fading channels, such as with stations in fixed locations that are nearby and without significant atmospheric or electromagnetic interference.
To summarize, assume that a channel pre-equalization system includes a feedback channel and that the channel fading is very slow. In a time division duplex (TDD) system, Channel Status Information (CSI) for a communication channel between a receiver and a transmitter can be estimated at the receiver. Then, the same CSI can be used to estimate the channel condition from the receiver to the transmitter due to channel reciprocity. For other duplex systems, such as frequency division duplex (FDD) system, the CSI can be estimated at the receiver side and then communicated via an explicit feedback channel. After the CSI is estimated, the signal to be transmitted can be pre-distorted by a pre-equalizer at the transmitter. The pre-distorted signal then travels through the channel such that a compensated signal is received at the receiver. Therefore, by virtue of the pre-equalization, there will be no net ISI caused by the channel and no need for equalization at the receiver.
However, since the pre-equalizer is a quasi-static filter (i.e., the coefficients are fixed for the duration of a single transmission), any change in the channel characteristics will not be compensated for, and, as a result, ISI will not be eliminated completely. It is possible in some circumstances to reduce this residual ISI by using previous CSI to predict future channel states. However, since practical channels are never entirely deterministic, a prediction error will inevitably degrade the performance of the pre-equalization process.
An improved method of signal pre-equalization is therefore needed that eliminates many of the disadvantages of the above-described prior art.